This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
The present invention relates to a valve assembly, in particular to a valve assembly having a flow control element of the plug and cage type arrangement, and to the use of the valve assembly in the processing of fluid streams. The present invention further relates to a wellhead assembly comprising the valve assembly, in particular a subsea wellhead assembly.
Valve assemblies are well known and widely used in an extensive range of fluid processing applications. Valves are used to control the flow of a fluid stream, for example to control the flow rate and/or pressure of the fluid stream.
One common valve assembly comprises a flow control element having a so-called ‘plug and cage’ arrangement. This assembly has a cage, typically cylindrical in form, comprising a plurality of holes or apertures therethrough for the passage of fluid. A plug, again generally cylindrical in form, is provided so as to be moveable with respect to the cage, the plug being disposed to be moveable to cover or close the apertures in the cage. The plug may be moved with respect to the cage between a closed position, in which all the apertures in the cage are covered, thus preventing fluid from flowing through the choke assembly, and a fully open position, in which all the apertures in the cage are open and available for fluid flow. Moving the plug with respect to the cage from the closed to the fully open position progressively uncovers the apertures in the cage, thus increasing the cross-sectional area available for fluid flow. In this way, the flow rate and pressure of the fluid may be varied and controlled. In the closed position, the end sealing portion of the plug contacts a seat formed in the choke assembly, so as to provide a fluid-tight seal, preventing the passage of fluid past the plug and cage. The plug may be arranged coaxially within the cage or coaxially exterior of the cage, known in the art as an external sleeve.
FR 2 436 922 discloses a valve for controlling the flow of a fluid, the valve comprising a housing having an inlet for fluid. The inlet is disposed in the wall of the housing and fluid flow through the inlet is controlled by a vane pivotably mounted at one side of the opening. Fluid leaves the housing through a centrally disposed outlet.
A fluid flow choke is disclosed in GB 2 124 341 and has a stationary tubular flow nozzle with throttling ports formed in a side wall thereof. A moveable throttling ring is moved over the nozzle to restrict or close the ports.
DE 36 15 432 concerns a flow valve having a slide gate moveable to align with a flow opening. The slide gate has an opening therein that has a cross-sectional area matched to that of the flow opening, such that a substantially constant response sensitivity is obtained across the range of settings of the valve.
A balanced sleeve control choke is described and shown in U.S. Pat. No. 5,086,808. The choke has a cage provided with openings therein and an exterior sleeve moveable over the cage to open and close the openings, thereby controlling the flow of fluid through the cage.
A valve having a closure member for creating turbulence in the flow of liquid through the valve is disclosed in U.S. Pat. No. 5,617,896.
U.S. Pat. No. 5,979,558 discloses a variable choke for use in subterranean well. The choke comprises an inner sleeve with openings therein, moveable with respect to an outer sleeve.
An apparatus and method for controlling the flow of a fluid is disclosed in WO 01/02697. The apparatus comprises a valve assembly having an orifice therein through which fluid flows. The orifice is shaped to provide a substantially consistent or otherwise predetermined change in pressure drop and flow rate between different positions of the valve. This is achieved by the orifice being shaped to provide a non-linear variation in the flow area throughout several positions of the valve.
US 2002/0020534 discloses a flow control device having inner and outer sleeves moveable with respect to each other and both provided with openings therein for the flow of fluid. The fluid flow is controlled by appropriate positioning of the inner and outer sleeves relative to each other.
A well choke is disclosed in US 2007/0095411. The choke is of a plug and cage design and has a quick closure device mounted between an actuator and the choke.
A flow control valve for gaseous or liquid media is disclosed in DE 37 17 128. The flow control valve comprises a plug and cage assembly arranged within a fluid inlet chamber. The plug and cage assembly is offset from the central longitudinal axis of the inlet chamber. The cage comprises a plurality of openings arranged in the wall of the cage. One portion of the cage comprises six openings extending horizontally through the cage, that is perpendicular to the longitudinal axis of the cage, at a tangent to the inner surface of the cage. Fluid flow through the openings in the cage is controlled by the position of the plug within the cage. A delivery chamber is disposed immediately downstream of the cage and is provided with a retarder to stop rotational flow of the fluid and produce a linear fluid flow pattern leaving the device.
More recently, EP 2 386 717 discloses a valve assembly of the plug and cage arrangement. The cage of the valve assembly is provided with a plurality of apertures therein, with the flow of fluid through the cage being controlled by the position of the plug within the cage. The apertures in the cage are arranged into rows, each row having one or more apertures. The rows of apertures are separated by lands, in which no apertures are present. In this way, the plug may be positioned within the cage such that its end face aligns with a land and does not extend across an aperture. In this position, each aperture is either fully open or fully closed. An advantage of this arrangement is that the erosion of the end face of the plug is significantly reduced. The apertures may extend tangentially to the inner wall of the cage.
EP 2 386 717 also discloses for the first time the formation of fluid bands within the cage of the valve assembly by fluid entering through the apertures. Each row of apertures can form a band of rotating fluid within the cage. The fluid bands act as hydraulic chokes, throttling the flow of fluid in the downstream direction within the cage.